It is known practice to dye keratin fibres, and in particular human hair, with dye compositions containing oxidation dye precursors, which are generally known as oxidation bases, such as ortho- or para-phenylenediamines, ortho- or para-aminophenols and heterocyclic compounds. These oxidation bases are colourless or weakly coloured compounds, which, when combined with oxidizing products, may give rise to coloured compounds via a process of oxidative condensation.
It is also known that the shades obtained with these oxidation bases can be varied by combining them with couplers or coloration modifiers, the latter being chosen especially from aromatic meta-diaminobenzenes, meta-aminophenols, meta-diphenols and certain heterocyclic compounds such as indole compounds.
The variety of molecules used as oxidation bases and couplers allows a wide range of colours to be obtained.
The “permanent” coloration obtained by means of these oxidation dyes must moreover satisfy a certain number of requirements. Thus, it must have no toxicological drawbacks, it must allow shades to be obtained in the desired intensity and it must show good remanance with respect to external agents such as light, bad weather, washing, permanent-waving, perspiration and rubbing.
The dyes must also allow grey hair to be covered, and they must be as unselective as possible, i.e. they must produce the smallest possible coloration differences along the same length of a keratin fibre, which is generally differently sensitized (i.e. damaged) between its end and its root.